In general, mobile communication systems have been developed to allow the user to communicate while moving around. The mobile communication system, fueled by rapid development of technologies, has reached a stage of providing high-speed data communication services as well as voice communication. In recent years, as one of next generation mobile communication systems, Long Term Evolution-Advanced (LTE-A) is being standardized by the 3rd Generation Partnership Project (3GPP). LTE-A is a technology to integrate cells with different frequencies supported in an e-NB to implement high-speed packet based communication that has higher transmission rate than a data rate currently provided, i,e., a carrier aggregation technology. As the 3GPP standard evolves, a small cell e-NB for supporting a cell with a small service area in the mobile communication system has often been used to expand capacity and eliminate shadow areas. The Release 12 (Rel-12) of the communication standard proposes a standard for providing high-speed communication services by integrating a cell of the small cell e-NB and a cell of the existing large cell e-NB, i.e., dual connectivity or inter-eNB carrier aggregation. Unlike the carrier aggregation provided by integrating a plurality of cells in a single e-NB as provided by LTE-A, high-speed uplink/downlink data services may be provided for terminals by integrating a plurality of cell carriers of heterogeneous e-NBs, according to the Rel-12.
The double connection has connectivity to two different e-NBs at the same time unlike the existing Rel-10 carrier aggregation, and may thus have the respective independent Physical Uplink Control Channels (PUCCHs). In the existing Rel-10 carrier aggregation, to send control information such as Channel Quality Indicator (CQI) or a response to data received at the UE (e.g., acknowledgment ACK/NACK), a control channel of a primary cell (PCell) is shared by secondary cells (SCells). Accordingly, it has a structure in which ACK for downlink data of the carrier aggregated SCells is delivered on an uplink control channel of the PCell. As for a random access channel, in the existing carrier aggregation, since the PCell and the Scell are serving cells included in the same e-NB, they may use the same uplink timing that the UE has. Accordingly, separate random access procedures for the SCell does not need to be performed. However, as for carrier aggregation for a distant small cell using the Remote Radio Head (RRH) as proposed by the Rel-11, it may follow the existing carrier aggregation scheme in which a small cell and a macro cell are included in the same e-NB, but requires separate random access because the distances from the UE to the macro serving cell and the distant small cell are different. In this regard, in the LTE Rel-11 standard, UE is instructed to configure a Timing Advance Group (TAG) based on the uplink timing for a serving cell and to control the uplink timing with Medium Access Control (MAC) control elements (CEs) for each TAG. Likewise, even in carrier aggregation between heterogeneous e-NBs in the Rel-12, a small cell and a macro cell are likely to have different uplink timing. However, since the small cell e-NB may have an uplink control channel in the carrier aggregation between heterogeneous e-NBs, the UE may control the uplink timing for serving cells of the small cell e-NB through a direct random access procedure. Therefore, definition for a random access procedure for a serving cell of a small cell e-NB in a mobile communication system is required.